RtpTransceiver bindings & requirements for publishing tracks (#39)

- RtpSender
- RtpReceiver
- RtpTransceiver
- RtpParameters
- VideoFrameBuilder
- Interior mutability on c++ side
- Cleanup bindings ( Use #pragma once instead of include guards )
     - webrtc-sys/src/* headers are now used in only one place 
     - Avoid confusion between generated headers and our headers
- AdaptedVideoTrackSource
- Cleanup the workaround with unsupported Vec<Shared<T>>
    - Put everything inside one file

webrtc-sys/src/peer_connection_factory.cpp

@@ -16,7 +16,6 @@
 #include "livekit/video_decoder_factory.h"
 #include "livekit/video_encoder_factory.h"
 #include "media/engine/webrtc_media_engine.h"
-#include "webrtc-sys/src/peer_connection_factory.rs.h"
 
 namespace livekit {